While presses according to the instant invention may have utility in regard to injection molding and reaction injection molding (RIM) processes, a particular area of applicability resides in the production of reinforced plastic parts or products that are composite structures in which a resin, either thermosetting or thermoplastic, is combined with a reinforcing member to strengthen and improve various properties of the plastic matrix. Particularly useful processes include resin transfer molding (RTM) and structural reaction injection molding (S-RIM). The former usually employ epoxies or polyester resins, although other thermosetting resins can also be employed. Polyurethanes are employed in S-RIM systems.
Both processes commonly employ a fiber preform or mold charge blank which may approximate the shape of the composite products or at least provides a structural backbone for the product. The mold charge blank is placed inside of a multiple-piece mold, after which the liquid resin or reaction components are transferred into the closed mold to impregnate the reinforcement material and fill all void space within the mold. After the necessary reaction and cure time for the particular resin, the mold is opened, and the finished composite product is removed.
Such composite products are currently of particular interest and importance in a variety of industrial applications, such as the automotive industry, where features such as weight reduction, strength, appearance, durability, flexibility, and consolidation of parts are significant design considerations. Automobile bumpers, for instance, made of polyurethanes reinforced with fibers from a mold charge blank, provide improved performance characteristics, particularly strength, and may also reduce the total weight and number of parts required.
Fiber mold charge blanks are manufactured separately and are supplied to the mold for combination with the liquid resin component(s). A variety of methods have been employed for making mold charge blanks, including the spraying of chopped fibers onto a molded surface and the forming of reinforcing mat or fabric in desired thickness and layers into the shape of the mold charge blank. The fiber material is normally combined with a thermoplastic binder material, and the "laid up" material is then placed in a preheat oven, transferred to a mold, pressed by the mold to the desired shape, and cooled to produce the actual mold charge blank which can be handled and stored until final manufacture. The binder material stiffens the mold charge blank, giving it appropriate structural integrity for liquid molding in the forming tool or handleability for storage or transfer to the final mold where resin transfer occurs.
Advances in automating the production of mold charge blanks have made the final molding process a primary limiting factor in the productivity of relatively large products requiring molds having projected area dimensions which may be on the order of 50 square feet. The final molding process is severely limited in a time sense due primarily to the reaction and cure time of the resins which may be on the order of ten seconds to a few minutes. Further, due to the size, weight, and complexity of the molds, most presses for such relatively large products employ very large and highly expensive hydraulic systems. Such hydraulic systems are necessary in order to achieve acceptable press closure speeds and to provide sufficient clamping forces over the extensive mold areas such that mold flash is eliminated or is reduced to a minimum.
Although the prior art relating to molding presses of this general type is extensive, very little of the known art appears to be directed to the combination of factors involved in achieving the molding of large parts at high production rates. For small parts, double-acting horizontal presses have been proposed employing a pair of spaced, fixed bases with a pair of bolsters supported on tie rods positioned outwardly of the bases for alternately opening and closing two molds. Horizontal presses having two bases and three bolsters to operate two molds independently by complex hydraulic controls have also been proposed. Both horizontal and vertical presses for simultaneously opening and closing two molds by a variety of mechanical arrangements are also known in the art. Other presses have employed complex hydraulic systems with large numbers of cylinders or complex fluid transfer systems to accommodate larger clamping force requirements. None of these approaches, however, lend themselves to the production of large parts at high production rates in a press configuration having flexibility in operation, reasonable size, and an attractive manufacturing cost.